As shown in FIG. 8, the phase system of a known motor servo apparatus includes a reference phase generation circuit 1, a delay circuit 2, and a phase error detection circuit 3.
The reference phase generation circuit 1, as shown in FIG. 9(a), generates a pulse array as a reference phase signal 4, which is applied to the delay circuit 2. The operation of this delay circuit 2 will be described here with the rising point of the reference phase signal 4 as a reference phase.
The delay circuit 2 includes two monomultivibrators 5 and 6, which produce a gate signal by delaying the reference phase signal 4 by a predetermined time period, and supply this delayed signal to the phase error detection circuit 3.
The phase error detection circuit 3 includes a trapezoidal wave generation circuit 9 and a sample and hold circuit 10. The trapezoidal wave generation circuit 9 generates a trapezoidal signal 11, as shown in FIG. 9(b), as a comparative operation waveform, with an operation starting point 12 of a slope region, which is the dynamic operation range, coinciding with the rising point of the gate signal 8. The sample and hold circuit 10 receives a PG signal 18 of 1 pulse per 1 turn as shown in FIG. 9(c) from a rotation phase detector 17 of a motor 16. The sample and hold circuit 10 samples and holds the value of the trapezoidal signal 11 at every rising point of the PG signal 18, which is outputted as a phase error signal 13.
By applying the phase error signal 13 to a motor drive circuit 15 through an appropriate phase compensation circuit 14, the rotation phase of the motor 16 is controlled so that the rising point of the PG signal 18 coincides with the rising point of the reference phase signal 4.
In this case, the phase error signal 13 must be symmetrical about an operational center 20 in the dynamic operation range 19 of the trapezoidal signal 11, according to changes in the rotation phase of the motor 16. Otherwise, a servo lock will come off, or, the phase system will not be servo locked. Furthermore, when the servo lock comes off due to an external perturbation, the servo lock becomes delayed, or, even if the phase system is servo locked, a deviation of the steady position occurs.
Therefore, a time constant of each of the monomultivibrators 5 and 6 of the delay circuit 2 is previously adjusted and fixed. In this case, the first monomultivibrator 5 is for phase adjustment, which generates a Q output 7 with a constant pulse width, as shown in FIG. 9(d), at every rising point of the reference phase signal 4. The first monomultivibrator 5 then applies the output 7 to the second monomultivibrator 6. The second monomultivibrator 6 is for so-called trapezoidal position determination, which generates a Q output with a constant pulse width as shown in FIG. 9(e) at every rising point of the Q output 7 of the first monomultivibrator 5, which is applied as the gate signal 8 for determining an operation starting point 12 to the trapezoidal wave generation circuit 9. During the time when the gate signal is at a high level "H," the trapezoidal signal 11 is at a lower limit value of the dynamic operation range. Furthermore, during the time when, after the dynamic operation is completed for a constant period T.sub.1, the gate signal 8 is at a low level "L," the trapezoidal signal 11 is at an upper limit value of the dynamic operation range.
Therefore, in the conventional art device shown in FIG. 8, if the reference phase signal 4 is a predetermined one with a constant period, the operational center 20 of the phase error signal 13 coincides with the center of the dynamic operation range 19, and phase control is performed about the operational center 20.
However, when the reference phase generation circuit 1 is of a variable period type, or is operated by an external signal 21 to generate a reference error signal 11 with an unknown or indefinite period, the operational center of the phase error signal 13 exceeds the center of the dynamic operation range. Furthermore, in an extreme case, the operational center occurs at, or approaches, the upper or lower limit of the trapezoidal signal 11.
FIG. 10 shows an operation timing diagram when the period of the reference phase signal 4 becomes longer than that shown in FIG. 9(a) for example. In this case, since the operational center 20 shifts from the center 22 of the dynamic operation range 19 to near the upper limit, the operational center 20 is considered to decrease the gradient of the dynamic operation range of the trapezoidal signal 11. However, since the gradient of the dynamic operation range is a key point in determining the servo gain, the gradient cannot be moderated too much, in view of the gain which must be increased as much as possible.
For a reference phase signal in which the operational center occurs at, or approaches, the upper or lower limit of the trapezoidal signal 11, a time constant select circuit 23 has been considered to be provided, as shown in FIG. 8, in the trapezoidal position determination monomultivibrator 6, so that the servo lock does not come off.
However, this method increases the time constant select circuit 23, and in turn, the packaging space on the circuit board, the number of parts, and the production cost. Furthermore, since the time constant of the monomultivibrator 6 can only be re-set to the extent that the servo lock does not come off, it is not guaranteed that the operational center will coincide exactly with the center 22 of the dynamic operation range 19, which will thus cause a considerable deviation in the steady position.
As another measure, a method has been considered in which a time constant select circuit 24 is provided in the phase adjustment monomultivibrator 5, as shown in FIG. 8. In this case, however, the operational center approaches, to some extent, the center 22 of the dynamic operation range 19, but the phase difference between the reference phase signal 4 and the PG signal 18 is not removed.